1. Field of the Invention
The invention relates to a process for manufacturing a strip of aluminium or an aluminium alloy for electrolytically roughened lithographic printing plates, whereby the alloy is continuously cast as a strip and the cast strip is then rolled to final thickness.
2. Background Art
Lithographic printing plates made of aluminium, typically having a thickness of about 0.3 mm, exhibit advantages over plates made of other materials, only some of which are:
A more uniform surface, which is well suited for mechanical, chemical and electrochemical roughening.
A hard surface after anodising, which makes it possible to print a large number of copies.
Light weight.
Low manufacturing costs
The publication xe2x80x9cALUMINIUM ALLOYS AS SUBSTRATES FOR LITHOGRAPHIC PLATESxe2x80x9d by F. Wehner and R. J. Dean, 8th. International Light Metals Conference, Leoben-Vienna 1987, provides an summary of the manufacture and properties of strip for lithographic printing plates.
Today, lithographic printing plates are made mainly from aluminium strip which is produced from continuously cast slabs by hot and cold rolling, whereby the said process includes intermediate annealing. In recent years various attempts have been made to process strip-cast aluminium alloys into lithographic plates, whereby in the process of rolling the cast strip to its final thickness at least one intermediate anneal has been necessary.
The microstructure close to the surface of strip after it has been rolled to final thickness is decisive for achieving uniform roughening via electrolytic roughening and electrochemical etching.
Up to now it has not been possible to obtain an etched structure in lithographic plate starting from cast strip which is superior to that obtained from conventionally continuously cast ingot.
The object of the present invention is therefore to provide a process of the kind mentioned at the start, in which the strip, rolled to final thickness, exhibits an optimum microstructure for electrolychemical etching.
That objective is achieved by way of the invention in that the rolling to final thickness is performed with a thickness reduction of at least 90% and without any further heating.
Here xe2x80x9cwithout any heatingxe2x80x9d means that the cast strip, after leaving the gap between the casting rolls, is not supplied with any heat from outside the strip until the rolling to final thickness has been completed. If the cast strip, which exhibits a relatively high temperature for a certain time after emerging from the gap between the casting rolls, is to be rolled to final thickness a short time after casting, then the starting temperature for rolling may be increased, especially in the case of large strip thickness. In the case of small strip thickness, the processing represents rolling to final thickness by cold rolling, without intermediate annealing.
The thickness of the cast strip is preferably at most 5 mm, in particular at most 4 mm. An ideal microstructure is obtained if the thickness of the cast strip is at most 3 mm, in particular 2.5 to 2.8 mm.
In principle any strip casting method may be employed to produce the cast strip, ideally, however, rapid solidification and, simultaneously, hot forming in the roll gap is desired. Both of the last mentioned properties are provided e.g. by the roll casting method in which the alloy is cast in strip form between cooled rolls. In the further processing of the cast strip by cold rolling, the advantageous grain structure in the regions close to the surface resulting from rapid solidification is retained.
The continuous casting process enables high solidification rates to be obtained and, at the same time, very fine grain sizes in the regions close to the surface as a result of dynamic recovery immediately after the cast strip leaves the roll gap.
The further processing of the cast strip involves coiling the cast strip to a coil of the desired size. In the subsequent processing step the strip is cold rolled to a final thickness of 150-300 xcexcm in a cold rolling mill suitable for producing lithographic sheet.
The strip which has been solidified and partially hot formed in the roll gap is not subjected to any further heatingxe2x80x94this in order to prevent grain coarsening from occurring. If the thickness of the cast strip is, however, much greater than 3 mm, e.g. 7 mm, then it may be necessary for the cast strip to be subjected to a hot rolling pass immediately after leaving the roll gap before it is rolled to final thickness. To achieve an optimum grain structure, at the same time minimising costly processing steps, one should if possible cast to such a small thickness that a hot rolling pass can be dispensed with.
Cold rolling without intermediate annealing leads to a highly cold-formed structure with a high density of dislocations and hence to a preferred microstructure which guarantees uniform electrochemical attack on etching.
Apart from the advantage of uniform attack on etching, the strip manufactured according to the invention also exhibits excellent mechanical properties e.g. high strength which diminishes only insignificantly during the stoving of a photosensitive coating in the production of lithographic printing plates.
The strip manufactured according to the invention is equally suitable for etching in HCl and HNO3 electrolytes, whereby the advantages of the microstructure obtained are realised especially on etching in an HNO3 electrolyte.
In principle all of the aluminium alloys normally employed for making lithographic printing plates may be employed for producing strip according to the invention. Especially preferred for this purpose are alloys of the type AA 1xxx, AA 3xxx or AA 8xxx.
After electrolytic etching in an HNO3 electrolyte, lithographic printing plates made from the strip produced according to the invention exhibit an improved etched structure for the same energy consumption compared to that of conventionally produced printing plates
The advantage of a lithographic printing plate made according to the invention over a conventionally produced plate is also that after the stoving of a photosensitive coating e g. for 10 min at 250xc2x0 C., the printing plate made according to the invention exhibits higher strength.
The above mentioned advantageous microstructure in the region close to the surface of the strip arises essentially because of the rapid solidification at the surface. As a result of the rapid solidification, the second phase particles in the microstructure precipitate out in a very fine form and in high density. These particles act as the first centres of attack during etching, especially if the electrochemical roughening takes place in an HNO3 electrolyte. When the rate of solidification at the surface is fast, the above mentioned particles exhibit an average spacing of less than 5 xcexcm and form therefore a continuous network of uniform points of attack at the surface. The growth of the actual three-dimensional roughness pattern starts from these first, uniform and highly numerous points of attack distributed over the whole surface of the strip. The small size of the mentioned intermetallic phases has the additional advantage that they considerably shorten the time required for electrochemical dissolution at the start of etching, as a result of which electrical energy can be saved. As non-equilibrium phases are formed by way of preference close to the surface of the strip during the rapid solidification according to the invention, the rate of dissolution of the mentioned fine particles is again higher than the rate of solution of the coarse intermetallic phases of equilibrium composition such as are formed in conventionally processed materials.
A further essential microstructural feature of the strip manufactured according to the invention is the small grain size formed during strip casting. The high density of points of penetration of the grain boundaries at the surface, together with a high density of vacancies in the grains themselves, leads to chemically active points of attack that continuously create new etching troughs.
The described microstructure at the surface of the strip leads to a significant improvement in the chemical etching process that creates the uniform roughness pattern required of lithographic printing plates. The advantages gained by using the strip produced according to the invention are as follows:
uniformly etched structure as a result of a high density of points of attack at the surface
etching n an HNO3 electrolyte under critical electrochemical process conditions
extending the etching parameters into the range of lower charging densities, thus saving electrical energy
preventing etching errors in HNO3 electrolytes due to undesired passivation reactions
forming a dense network of cracks in the oxide layer in the passivation range of the anodic potential via a high density of small intermetallic particles of non-equilibrium structure
forming a dense network of vacancies in the natural oxide skin in the passivation range of the anodic potential as a result of a small grain size with many points where the grain boundaries penetrate the oxide layer.
The advantage of a strip material produced according to the invention over strip material conventionally manufactured is seen in the following summary of test results relating to the surface condition of the strip surface which, as explained above, has a decisive influence on etching behaviour. The improved etching behaviour of the printing plates manufactured according to the invention over conventional printing plates is explained by way of two examples which are documented by scanning electron microscope photographs which show at a magnification of 1000 times in